Exhaust purifying system for internal combustion engine

Abstract

An object is to provide an exhaust purifying system for an internal combustion engine having an NSR catalyst, which can inhibit emissions from being aggravated by blow-by of NOx. An exhaust purifying system for an internal combustion engine capable of a lean burn operation is provided. The exhaust purifying system includes: an NSR catalyst disposed in an exhaust passage of the internal combustion engine; an SCR disposed downstream of the NSR catalyst; a NOx sensor disposed downstream of the SCR, the NOx sensor producing an output according to an NH 3 concentration; and rich spike means for performing a rich spike. The rich spike means starts the rich spike at a predetermined timing during a lean burn operation and terminates the rich spike at a timing when the NOx sensor issues a predetermined output characteristic indicative of a rise in the NH 3 concentration.

The invention claimed is: 
     
       1. An exhaust purifying system for an internal combustion engine capable of a lean burn operation, the exhaust purifying system comprising:
 an NOx storage-reduction catalyst (hereinafter referred to as an “NSR catalyst”) disposed in an exhaust passage of the internal combustion engine; 
 an NOx selective catalytic reduction (hereinafter referred to as an “SCR”) disposed downstream of the NSR catalyst; 
 an exhaust gas sensor disposed downstream of the SCR, the exhaust gas sensor producing an output according to an NH 3  concentration; 
 an electric control unit programmed to perform a rich spike, wherein the electric control unit is further programmed to start the rich spike at a predetermined timing during a lean burn operation and terminates the rich spike at a timing when the exhaust gas sensor issues a predetermined output characteristic indicative of a rise in the NH 3  concentration; 
 a second exhaust gas sensor disposed downstream of the NSR catalyst and upstream of the SCR, the second exhaust gas sensor producing an output according to the oxygen concentration, 
 wherein the electric control unit is further programmed to: 
 perform a first rich spike at a predetermined air-fuel ratio; 
 perform a second rich spike at an air-fuel ratio leaner than the predetermined air-fuel ratio; and 
 switch operation mode from the first rich spike to the second rich spike at a timing when the second exhaust gas sensor issues a predetermined output characteristic indicative of a reduction in the oxygen concentration when the internal combustion engine runs at heavy load. 
 
     
     
       2. The exhaust purifying system for an internal combustion engine according to  claim 1 , wherein
 the SCR is an Fe-based zeolite catalyst. 
 
     
     
       3. The exhaust purifying system for an internal combustion engine according to  claim 1 , further comprising:
 a second NOx selective catalytic reduction (hereinafter referred to as a “second SCR”) disposed downstream of the exhaust gas sensor. 
 
     
     
       4. The exhaust purifying system for an internal combustion engine according to  claim 3 , wherein
 the second SCR is an Fe-based zeolite catalyst. 
 
     
     
       5. An exhaust purifying system for an internal combustion engine capable of a lean burn operation, the exhaust purifying system comprising:
 an NOx storage-reduction catalyst (hereinafter referred to as an “NSR catalyst”) disposed in an exhaust passage of the internal combustion engine; 
 an NOx selective catalytic reduction (hereinafter referred to as an “SCR”) disposed downstream of the NSR catalyst; 
 an exhaust gas sensor disposed downstream of the SCR, the exhaust gas sensor producing an output according to an NH 3  concentration; 
 an electric control unit programmed to perform a rich spike, wherein the electric control unit is further programmed to start the rich spike at a predetermined timing during a lean burn operation and terminates the rich spike at a timing when the exhaust gas sensor issues a predetermined output characteristic indicative of a rise in the NH 3  concentration; 
 a three-way catalyst disposed upstream of the NSR catalyst; and 
 a second exhaust gas sensor disposed upstream of the NSR catalyst and downstream of the three-way catalyst, the second exhaust gas sensor producing an output according to an oxygen concentration: 
 wherein the electric control unit is further programmed to: 
 perform a first rich spike at a predetermined air-fuel ratio; 
 perform a second rich spike at an air-fuel ratio leaner than the predetermined air-fuel ratio; and 
 switch operation mode from the first rich spike to the second rich spike at a timing when the second exhaust gas sensor issues a predetermined output characteristic indicative of a reduction in the oxygen concentration when the internal combustion engine runs at light load. 
 
     
     
       6. The exhaust purifying system for an internal combustion engine according to  claim 5 , wherein
 the SCR is an Fe-based zeolite catalyst. 
 
     
     
       7. The exhaust purifying system for an internal combustion engine according to  claim 5 , further comprising:
 a second NOx selective catalytic reduction (hereinafter referred to as a “second SCR”) disposed downstream of the exhaust gas sensor. 
 
     
     
       8. The exhaust purifying system for an internal combustion engine according to  claim 7 , wherein
 the second SCR is an Fe-based zeolite catalyst.